1. Technical Field
This invention relates generally to various silicon-containing products and to methods for making such products from starting materials obtained from plant matter or processed plant matter having a silica content greater than about three weight percent, most preferably rice hulls and rice straw. One aspect of the invention relates to the production of silicon, silicon carbide, silicon nitride, silicon tetrachloride and other carbon-silica products from silica-containing plant matter such as rice hulls and rice straw. Another aspect of the invention relates to silicon-containing products having total mineral impurities lower than 1,000 ppm and phosphorus contents lower than 400 ppm. Another aspect of the invention relates to photovoltaic-grade silicon made from rice hulls and rice straw. Another aspect of the invention relates to intermediate carbon-silica products as defined herein that have low mineral contents previously unattainable using leaching processes and desired ratios of fixed carbon to silica. Another aspect of the invention relates to high purity carbon-silica products as defined herein that are made by leaching silicon-containing plant matter with sulfuric acid for controlled periods at controlled temperatures to achieve desired ratios of fixed carbon to silica. Another aspect of the invention relates to chemical and thermal methods for removing and recovering volatile carbon from compositions made by the leaching process, and to the resultant devolatilized carbon-silica products. Another aspect of the invention relates to silicon-containing products such as silicon, silicon carbide and silicon nitride made from leached and devolatilized, high purity, carbon-silica products using a carbothermal process.
2. Description of Related Art
The unique performance properties of high purity silicon have made the development of the semiconductor industry possible and are important to the rapidly growing photovoltaic industry. Other well known silicon-containing materials include, for example, silicon carbide and silicon nitride, which, in certain forms, are used to produce high performance ceramics and high performance composites. These, and other silicon-containing materials, are used in a variety of applications including electronics, defense, automotive, aerospace, industrial wear parts, advanced glasses, and in chemical and environmental products.
The photovoltaic industry is growing at a rapid pace, but the cost of silicon is one of the deterrents to even faster growth and to the production of bulk power using photovoltaic panels. Due to a lack of cost-effective processes for making solar-grade silicon, a majority of solar cells are presently made from the more pure and more costly semiconductor-grade silicon. Likewise, the demand for high performance ceramics and composites is growing, but the promise of these industries is hampered by the high cost of materials such as silicon carbide and silicon nitride.
The production of essentially all materials and products containing silicon involves the reaction of carbon and silica (SiO2) at a very high temperature, often referred to as a carbothermal reduction. The carbon “pulls” oxygen atoms off the silica and the resulting carbon monoxide exits the reactor as a gas, leaving behind the silicon product. If the desired product is silicon by itself, then the molar ratio of fixed carbon to silica for this reaction should be 2:1 as shown below:
If the desired product is silicon carbide (SiC), the molar ratio for the reaction should be approximately 3:1, as shown by the following formula:
Other Si-based products, such as ferrosilicon (FeSi) and silicon nitride (Si3N4) and silicon tetrachloride (SiCl4) can be produced using the same reactions between carbon and silica. For example, to make silicon nitride, two moles of carbon are used per mole of silica, while the reaction is carried out under a nitrogen-containing atmosphere.
The standard commercial process for producing silicon involves the mixing of a carbon source such as coke with crystalline silica, i.e. sand or quartz, in a predetermined ratio and reacting this mixture to produce metallurgical grade silicon. Because the coal and sand particles are large and not very porous, with limited contact and available surface area, the rate of reaction using the conventional feedstocks is slow, typically taking more than a day to complete. As a result, the energy requirements for this carbothermal process are high, and the silicon produced is often less than 99% pure. Silicon produced in this manner is suitable for applications in the aluminum and certain chemical industries, but is not of adequate purity for such applications as the semiconductor and photovoltaic industries. Very expensive and complicated processes have previously been needed to upgrade the metallurgical silicon to photovoltaic and semiconductor grade materials.
Similarly, silicon carbide produced using the slow and energy-intensive Acheson process is costly and is limited to use in the metallurgical, refractory and abrasives industries and in other applications that do not require high purity, small particle size, and/or whiskers or fibers. The production of high-end silicon carbide powders has previously required the use of processes that are even more expensive and complicated.
Silicon nitride can be produced through a variety of processes and is often made commercially using expensive starting materials such as silicon tetrachloride. The resultant powder, although commercially desirable, is too expensive for use in all but a few high-end applications.
During the past 20 years, several researchers have investigated the use of rice plants as a source of silica for industrial products. All plant matter contains significant amounts of carbon and many types of plants contain silica. The rice plant is perhaps one of the most unique because of its high concentrations of silica. Whereas the mineral content of most plants is, for example, about 1-2%, the rice plant typically has a mineral content of about 11-23%. More significantly, about 75-95% of the mineral content of the rice plant is silica. Rice straw contains about 11% silica and rice hulls typically contain about 15-23% silica. Although rice is one of the most abundant crops grown worldwide, to date there has been little practical use for rice hulls and rice straw, which constitute a considerable portion of the rice plant.
Rice hulls are the natural sheaths that form on rice grains during their growth. They are removed during the refining of rice and are a waste or low value by-product of the rice milling industry. Rice straw consists of stem, leaf sheathes, leaf blades and the remains of the panicle after harvesting. Generally, the amount of rice straw obtained from rice plants is at least equal to the rough yield of rice harvested. Because of their high silica content, these materials have little value as components of animal feeds. Because rice hulls and rice straw have a relatively large amount of potassium that interacts with the silica at combustion temperatures to produce boiler slag and deposits, and have a large fraction of noncombustible ash, they are similarly viewed as being a poor fuel source. If rice hulls and rice straw are burned as fuel, the relatively high ash content of both rice hulls and rice straw requires special handling equipment. For these reasons, rice hulls are frequently deposited in landfills and rice straw is usually burned in the fields. Thus, rice hulls and rice straw have little-to-no commercial value and have historically presented a disposal problem.
Nevertheless, rice hulls and rice straw continue to be an attractive source of silica because of the high silica content and low cost. Most of the organic material in rice hulls can be removed by combustion. The ash produced from such combustion processes comprises up to about 95% silica, but still contains non-silica mineral impurities. Several investigators have studied the removal of non-silica minerals from rice hulls in order to create value-added products from the “purified” hulls.
L. P. Hunt, J. P. Dismukes, J. A. Amick, “Rice Hulls as a Raw Material for Producing Silicon,” J. Electrochem. Soc., 131(7), 1984, investigated the potential use of rice hulls for producing silicon pure enough for fabrication into solar cells and low enough in cost so that photovoltaic energy could be more cost-competitive with conventional energy sources. Following grinding of raw rice hulls to −20 to +80 mesh, washing and drying, two samples of the dried hulls were leached for 15 minutes under boiling conditions with HCl:deionized water at 1:3 and 1:10, respectively. Leaching with 1:10 acid solution was found to be just as effective as 1:3 acid solution. However, leaching a sample with the weaker acid solution for 5 hours at 50° C. did not reduce impurity concentrations to levels as low as those attained under boiling conditions. Three rice hull samples from different sources were acid leached for 15 minutes in boiling 1:10 HCl acid solution. Concentrations of calcium, potassium, magnesium and manganese were reduced by factors of 40-100 times (97.5 to 99% removal). Sulfur concentration was reduced by a factor of 8 (87.5% removal); sodium and phosphorus concentrations were reduced by a factor of about 3 (67% removal); boron, aluminum and iron concentrations were not reduced. The investigators reported that raw rice hulls have a total non-silica mineral impurity concentration about 30 times greater than that of leached hulls (overall 96.7%), and projected that an acid-leached and coked product is of interest as a raw material for the production of solar-grade silicon. However, it was noted that since the phosphorus/boron ratio exceeded 10, eventual fabrication of solar cells would require a different process to substantially reduce the concentration of phosphorus, which was reported at 40 ppm (averaged).
The effect of porosity on encouraging the production of silicon carbide whiskers as well as the importance of porosity in removing mineral impurities is disclosed in U.S. Pat. No. 4,504,453 (1985) to Tanaka.
M Patel, A. Karera and P. Prasanna, “Effect of thermal and chemical treatments on carbon and silica contents in rice husk,” J. Mater Sci, 22 (7), 1987, report subjecting rice husk samples obtained from the vicinity of Bhopal, India, to treatment with laboratory grade hydrochloric (4 to 12 N), sulfiric (2 N) or nitric (8 N) acid for 2 to 6 hours at 100° C. The authors concluded that SiO2 of 99% purity (10,000 ppm of non-silica minerals, which is a high impurity level for high purity applications) can be produced from rice husk treated with HCl, followed by carbonization at temperatures below 700° C. to avoid any transformation of amorphous to crystalline form, and that the purity cannot be increased above 99% because the remaining 1% may be metal oxides insoluble in acid. They also reported that reflux reactions are difficult to carry out in sulfuric or nitric acid, and therefore only limited experiments were performed. The authors also reported that they were able to achieve a carbon:silica ratio of 2:1 by partial coking or rice hulls. However, this was a mass ratio that corresponds to a mole ratio of 10:1, whereas the desired molar ratios are in the range of 3:1 to 2:1.
A. Chakraverty, P. Mishra and H. D. Banerjee, “Investigation of combustion of raw and acid-leached rice husk for production of pure amorphous white silica,” J. Mater Sci, 23(1), 1988, disclosed milling cleaned and dried rice husks to a particle size of about 40 mesh, leaching in hydrochloric (1 N, 3 N, 5 N and 11.3 N), sulfuric (1N, 4.5 N, 9 N and 18 N) and nitric (4.5 N, 9N and 18N) acid at 50° C. for 2 hours, washing with distilled water, and drying. The acid-leached husks were then combusted at temperatures ranging from 500 to 700° C. Acid treatment with sulfuric acid was less effective than leaching with either hydrochloric acid or nitric acid at comparable concentrations for reducing the concentrations of oxides of sodium, potassium, calcium, iron, magnesium, manganese, zinc and copper. The overall metallic impurity level (reported as oxides) ranged from 300-747 ppm for samples treated with hydrochloric acid, from 496-688 ppm for samples treated with nitric acid, and from 3534-4483 ppm for samples treated with sulfuric acid. Even the best performance, achieved using 11.3 N HCl, failed to produce a 99% reduction in the identified non-silica impurities. No data was presented regarding phosphorus levels either before or after treatment.
R. Conradt, P. Pimkhaokham and U. Leela-Adisorn, “Nano-structured silica from rice husk”, J. of Non-Crystalline Solids, 145 (1992) 75-79, report the acid leaching of washed Thai rice husk by reflux boiling in 2.4 molar hydrochloric acid or 3.6 molar sulfuric acid for 3 hours at a ratio of 100 g. husk/liter. The leached husks were subsequently incinerated at 600° C. and characterized. The investigators concluded that omission of acid pre-treatment yielded a considerably reduced surface area in the incinerated products. Specific surface areas of 180-250 sq. m/g. are reported for the silica prepared from rice husks. The best non-silica mineral level achieved in the ash obtained from rice hulls pretreated with HCl was 6500 ppm. The best non-silica mineral purity level in the ash obtained from rice hulls pretreated with sulfuric acid was 10,000 ppm.
I. A. Rahman, “Preparation of Si3N4 by Carbothermal Reduction of Digested Rice Husk,” Ceramics Int'l (1994), investigated the production of carbon and silica through digestion of rice husk using nitric acid. About 30 g of prewashed rice husk was digested in 300 ml of nitric acid at 60° C. The concentration of acid was varied from 10M to 14M. During digestion (for up to 7 hrs. with continuous stirring), the temperature was carefully controlled. The digested husk was then filtered and washed with distilled water until neutral. The overall non-silica mineral level (reported as oxides) in the digested husks was reported to be 2,500 ppm. The digested husk was pyrolysed at 800° C. to a constant weight under a flow of argon gas. The carbon content of pyrolysed digested husk was determined by heating at 700° C. in air for one hour. Rahman reported that after more than 3 hours digestion in 12M nitric acid, the digestion product obtained was in the stoichiometric ratio of 2C/SiO2 and that higher concentrations could not produce the desired ratio. Lower concentrations required a longer time to produce a suitable carbon-silica mixture. The weight loss after heating was considered to be the amount of carbon present in the pyrolysed digested husk, and the residue was considered to be pure silica. Next, the pyrolysed powder was nitrided by heating in a furnace in a controlled nitrogen atmosphere, raising the temperature gradually to 1430° C., after which the products were burned in a muffle furnace at about 700° C. for 30 min. to remove any excess carbon. The most important factor contributing to the completeness of reaction was found to be the homogeneity of mixing, and the use of rice husk was said to be an advantage, as the silica and carbon are naturally mixed.
C. Real, M. Alcala and J. Criado, “Preparation of Silica from Rice Husks,” J. Am. Ceram. Soc., 79(8) 1996, investigated a procedure for obtaining pure silica gel with a high specific surface area from rice husks and reported that silica with >99% purity can be obtained by burning rice husks at 600° C. under inert atmosphere, followed by the combustion of the residual carbon under oxygen atmosphere at the same temperature, provided that either the husks used as raw material or the silica obtained as final product have been leached previously in boiling 10% hydrochloric acid solution for 2 hrs. However, the yield of a silica gel with a high specific surface area and a homogeneous distribution of nanometric particles requires either the preliminary leaching of the rice husks with diluted hydrochloric acid or its washing with boiling water to remove the K+ cations.
R. V. Krishnarao and J. Subrahmanyam, “Formation of SiC from Rice Husk Silica-Carbon Black Mixture: Effect of Rapid Heating,” Ceramics Int'l, 22 (1996) 489-492, demonstrated that porosity is important for the production of silicon carbide whisker.
N. Yalcin and V. Sevinc, “Studies on silica obtained from rice husk,” Ceramics Int'l 27 (2001) 219-224, report leaching of washed and dried rice husks by reflux boiling in 3% (v/v) HCl or in 10% (v/v) sulfuric acid for 2 hours at a ratio of 50 g. husk/l, or by leaching with 3% (v/v) NaOH solution for 24 h at room temperature at a ratio of 50 g husk/l. After leaching the husk was thoroughly washed with distilled water, dried in an air oven at 110° C. and then burned in a muffle furnace at 600° C. by four different methods. The investigators found that the silica content of rice husk ashes was strongly dependent upon the type of chemical used for leaching the rice husks prior to incineration. Reported SiO2 content (wt %) was 99.60±0.05 (4000 ppm) for the ash samples where the husks were pre-leached with 10% (v/v) sulfuric acid (v/v) for 2 hr. at the boiling point, and 99.66±0.02 (3400 ppm) for the ash samples where the husks were both pre- and post-leached with 3% HCl (v/v) for 2 hr. at the boiling point. The sample leached with sulfuric acid before incineration exhibited a BET specific surface of 282 m2/g.
Except for Hunt et al and Rahman et al., the foregoing references do not suggest that the carbon found in rice hulls can be used as a source of carbon to be reacted with the silica in rice hulls to form desired products. Instead, the rice hulls are burned to form ash, thereby removing all of the carbon, after which the ash is treated with acid to remove minerals from the resulting silica. However, because the removal of K+ prior to heating the rice hulls allows the silica to maintain a higher specific surface area and smaller particle size upon heating, Real et al. in 1996 demonstrated that it is advantageous to carry out acid leaching prior to using combustion to reduce the rice hulls to silica. In either situation, the resulting silica is combined with carbon or another reducing agent from another source in order to produce the desired silicon-containing product.
Rice hulls contain both fixed and volatile carbon. Fixed carbon is retained in the solids to high enough temperatures to react with silica to form products, while volatile carbon is volatilized at relatively low temperatures, making it unavailable to react with the silica. The mole ratio of fixed carbon to silica in untreated rice hulls is about 4:1. Consequently, the mole ratio must be adjusted to the proper ratio for the desired product (e.g., about 3:1 for SiC and about 2:1 for Si). It is important to note that this is a mole ratio and is not a mass percentage ratio, as is sometimes used in the prior art. Even a mass percentage ratio as low as 1:1 correlates to a mole ratio of about 5:1, which is well above what is required in most carbothermal reactions. Hunt et al. referenced the use of pyrolysis followed by controlled combustion with CO2 to remove some of the fixed carbon after the mineral content had been previously reduced to moderate levels with HCl, which levels contained unacceptable amounts of phosphorus and no reduction of iron. Rahman et al. used concentrated nitric acid, which is a strong oxidizing agent that disintegrates organic material in the husk, to remove carbon. However, the mineral purity of the ash from the treated husks was still 2500 ppm, with an iron content at 200 ppm. No data was reported regarding phosphorus. As noted by Rahman et al., the high reactivity of the carbon after acid treatment, the retention of the high specific area of the silica after acid treatment and the intimate association of the silica and carbon in acid treated rice hulls should enhance the rate of reaction between the silica and the carbon at high temperatures.
To make materials that can cost effectively achieve the properties required for solar power and high performance materials, it is critical to develop processes that can achieve adjusted mole ratios of intimately associated fixed carbon and silica, and that can achieve very low levels of non-silica minerals, particularly phosphorus and iron, from plant material containing high levels of silica. These materials can then be used in less-expensive carbothermal processes to produce high-end silicon, silicon carbide, silicon nitride, silicon tetrachloride and other silicon-based products at a much lower cost than the current processes.